The present invention generally relates to automatic lensmeters and, more particularly to a lensmeter which has a mechanism suitable for find a measuring position for near vision at the time of measuring addition diopter of a progressive addition lens.
There have been so far proposed various sorts of lensmeter for automatically measuring various optical characteristics of ophthalmic lenses for use in spectacles. These lensmeters have measuring mechanisms which have different unique features and a common feature that a target having a predetermined pattern is provided on the object or image side of a lens to be measured so that a light receiving element detects the target image formed thereon, whereby optical characteristics of the lens are measured on the basis of the detected result.
As the number of elderly people has increased, the use of progressive addition lenses has recently increased in ophthalmic lenses for middle-age people. However, it often occurred that, since it is structurally difficult to find the suitable position for measuring optical powers in the near vision portion from its outside in the progressive addition lens, measurement is erroneously carried out in the wrong measuring position whereby an accurate addition diopter cannot be obtained. This is because, in the progressive addition lens, the measurement of refractive powers of the far vision portion is not especially difficult as a result of its broad area. However, marks (printed marks) indicative of measuring points for the far- and near-vision portions are erased after the lens is once processed, so that it becomes difficult to find the measuring position for the near vision portion.
There has been conventionally suggested a method of finding the near-vision measuring point with the use of a hidden mark indicative of the horizon of the lens and a template indicative of the measuring point. Even in this method, however, it is difficult to find the hidden mark because the lens has usually many scars on its surface. In addition, this method is defective in that, even if the hidden mark is found, it is hard to select a suitable one of such templates since they differ from manufacturer to manufacturer.
For the purpose of eliminating the above defects a positioning mechanism for accurately measuring addition diopter of a progressive addition lens has been proposed in JP-A-61-251732.
The proposed mechanism, considering that far- and near-vision portions in a progressive addition lens are different in spherical refractive power but are equal in cylindrical refractive power. The mechanism incorporates comparison means for outputting a positioning completion signal when S.sup.0 .noteq.S.sup.1 and C.sup.0 =C.sup.2 and a display unit driven by the completion signal.
According to this mechanism, an accurate measured value of the lens is obtained under measurement when an indication saying the positioning completion is displayed on the display unit. With the mechanism, however, since an operator can know only the fact that the positioning is completed, he cannot judge to what degree the area being now measured deviates from the near vision portion. Accordingly, this mechanism requires a considerable time before the positioning completion and also a considerably experienced operator.